In the conventional technologies, an indium film having a sea-island structure is provided on the surface of a substrate by vacuum deposition method to decorate a member required a penetration property of an electromagnetic wave. The indium film having a sea-island structure not only penetrate electromagnetic waves due to a sea-island structure but also has a sufficient metallic gloss as a decorative metal film because of an extremely fine sea-island structure. Therefore, such an indium film having a sea-island structure is used as a metal film that decorates the cover member (radome) of a vehicle-mounted millimeter wave radar devices, such as an emblem, for example (see “Patent Document 1” and “Patent Document 2”).
However, the vacuum deposition method consumes equipment cost for vacuum equipment and the like. Further, as the vacuum deposition method should form a film inside a vacuum chamber, restrictions including the size limitation of the substrate to be a size that can be housed in a vacuum chamber. In addition, as a substrate should be placed in a vacuum chamber for formation of a film once and again, mass productivity is poor.
In addition, a millimeter wave radar devices transmits a transmitting wave in a predetermined angle range, detects a direction where an obstacle presents according to the received wave based on an angle at which the received wave is detected and determines relative speed to the obstacle based on time lag from transmission of the transmitting wave to receiving of the received wave. In the operation, if the transmitting and receiving angles in the transmitted wave and the received wave shift due to attenuation of an electromagnetic wave in the metal film, accurate determination of the position and relative speed of the obstacle may not be achieved.